desktop-shell: Click top left and align clock

Pads launchers with the empty space that used to be around them. Moving
pointer to 0,0 and clicking launches the preferred app. First launcher
has more padding at its start to look nice.
Moves the clock to the right edge with same padding. Keeps one of the
two values for text extents that the code was already retrieving but
never read. Horizontal panel position centers the clock.
Sets text in the panel, meaning tooltips and the clock, to consistent 14
units of the default system font at 85% of the max brightness, so it's
less tiring on eyes.